The present invention is in a fluidized bed system for carrying out exothermic processes in a circulating fluidized bed. The system comprises a fluidized bed reactor, a solids separator and a return line. Additionally, the system includes lines for supplying oxygen-containing primary gases through the bottom of the fluidized bed reactor, lines for supplying oxygen-containing secondary gases on a level which is at least 1 meter above the bottom of the reactor but not in excess of 30% of the height of the reactor, and a fuel line, which opens into the fluidized bed reactor between the primary and secondary gas inlets.
Processes and plants which use circulating fluidized beds, particularly for a combustion of carbonaceous materials, have proved to be most advantageous and are superior for many reasons to processes and plants in which so-called orthodox fluidized beds are employed.
The basic process for combustion in a circulating fluidized bed has been described in U.S. Pat. No. 4,165,717. In that process the combustion is effected in two stages and the heat of combustion is dissipated via cooling surfaces disposed so as to contact the solids of the fluidized bed reactor above the secondary gas inlet. One of the special advantages of that process is that the combustion process can be adapted in a technically simple manner to the power requirement by the control of the suspension density in the upper portion of the reactor space and of the heat transfer to the cooling surfaces in contact with the fluid bed solids..
In U.S. Pat. No. 4,111,158 a combustion process is described in which a circulating fluidized bed also is employed. In that process a portion or all of the heat of combustion is extracted in an external fluidized bed cooler, which succeeds the fluidized bed reactor. The cooled solids are recycled in order to maintain a substantially constant temperature in the fluidized bed reactor. In the latter process an adaptation to the power requirement can be achieved by increasing or decreasing the rate at which solids are passed through the external fluidized bed cooler and then recycled to the fluidized bed reactor.
Whereas the processes outlined hereinbefore have proved highly satisfactory, the current trend toward increased power generation involve certain difficulties in the plant design. For larger thermal capabilities, larger reactor dimensions and particularly larger reactor cross-sections are required (e.g. above about 300 MW.sub.th corresponding to a reactor area of above about 50 m.sup.2.) Due to the larger reactor dimensions and/or reactor cross sectional area, it is difficult to achieve a satisfactory transverse mixing of fuel and the like and oxygen-containing secondary gas throughout the total area of the fluidized bed reactor adjacent to the inlet means. As a result, a considerable part of the reaction occurs in the upper portion of the reactor and an afterburning may undesirably take place after the solids and gas have been separated in the solids separator.
It is an object of the invention to provide a fluidized bed system which comprises a fluidized bed reactor, a solids separator and a return line and serves to carry out exothermal processes in a circulating fluidized bed and which ensures a satisfactory operation even during a generation of a high thermal power.